Many types of input devices are available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens, and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens generally allow a user to perform various functions by touching (e.g., physical contact or near-field proximity) the touch sensor panel using a finger, stylus or other object at a location often dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can generate touch images and then interpret the touch images in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch image.
Electronic devices in general can be susceptible to tonal noises that can be coupled to the device via proximal electronics such as an external charger. In the instance of a touch input device, tonal noise can generate a “false touch” on the screen; in other words, the device will determine that a touch or proximity event has occurred when none exists. In some cases, these tonal noises can additionally or alternatively cause error(s) in touch position calculations, such as jittering of the touch position with time, that can cause inaccuracies in touch input when accurate touch position calculation is important (e.g., when user input elements are spaced close together on the screen, such as in an on-screen keyboard). Furthermore, tonal noise can also cause a device to ignore an actual touch or proximity event. For example, mutual capacitance touch sensor panels can be formed from a matrix of drive and sense lines of a substantially transparent conductive material such as Indium Tin Oxide (ITO). The lines are often arranged orthogonally on a substantially transparent substrate. Tonal noise can be coupled into the matrix of drive lines and sense lines, causing signals to appear that can be misinterpreted as a touch or proximity event. Also, tonal noise can be coupled into the matrix of drive and sense lines causing signals to appear as negative touches, such that when a real touch occurs, it is not detected. The false touches or undetected touches can lead to an overall degradation of the user experience in that the device will register touches that the user did not intend, and furthermore, may fail to recognize actual touches intended by a user of the device.